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Hubble White Dwarf Images
Publication Date: Fall 2004
It isn’t a character from The Lord of the Rings, and it’s not the name of the latest punk band. A white dwarf supernova is, in fact, an exploding star. These phenomena have provided most of the Earth’s iron and helped scientists measure the size and age of the universe.
Recently, a group of astronomers identified the type of star system that causes white dwarf supernovas. The new information allows for a major leap in our understanding of these explosions. The team included ASU astronomers Sumner Starrfield and Steve Dwyer.
These massive explosions occur on a white dwarf star that is part of a binary—or double—star system. These supernovas appear to eject heavy elements like iron, but no hydrogen or helium.
Scientists consider that strange, because hydrogen and helium are the most abundant elements in the universe. They provide the nuclear fuel for stars. Although several types of binary star systems contain a white dwarf, they show evidence that large amounts of hydrogen and helium would be released if the white dwarf exploded.
Astronomers rely on models of star systems to make comparisons and develop theories. They found that current models of systems that produce white dwarf supernovas match observations of certain binary star systems called Super Soft X-ray Sources.
In this type of double star system, the white dwarf is extremely hot and bright. The second star is thought to be similar to our own sun. Gas from this star falls onto the white dwarf.
“Our calculations show that an Earth-size mass of material must be falling onto the white dwarf every few years,” Starrfield says. “At these high rates we find that hydrogen fuses to helium at the very surface of the white dwarf.”
Helium beneath the surface fuses to carbon and oxygen and possibly other elements. Eventually, the core of the white dwarf becomes so dense that an explosion occurs.
The astronomers say that because the hydrogen and helium have fused to heavier elements, they are not seen in observations of these supernovas. In addition to the ASU astronomers, the research team also included scientists from Oak Ridge National Laboratory, Los Alamos National Laboratory, and Villanova University. They presented their findings at the American Astronomical Society meeting in January 2004.—Diane Boudreau